Earth-observation satellites intend to image target areas of earth under clear sky conditions. The satellites that are designed for earth observation purposes are typically placed in a lower orbit than other satellites. Typical orbit altitude for this type of satellites is around 450 km. Due to space mechanics involved; any satellite located at such a low orbit has to revolve around earth much faster than the rotation of the Earth itself. Typical orbit time for such satellites is about 90 minutes which means that satellite will be completing it is rotation over earth every 90 minutes. Orbit inclination is an important factor, which determines whether the satellite will be orbiting the same path or visiting different paths as it revolves around globe. Satellites with orbit inclination 0° rotate over the equator all the time without going over any other location. Satellites with orbit inclination 45° go over areas of northern and southern hemisphere of Earth between 45° latitude. Satellites with orbit inclination 90° or more can fly over any part of the earth. Typically, this is what most commercial earth-observation satellites desire since they need the capability to image any part of the earth desired by their clientele. Most Earth observation satellites also fly in a “sun-synchronous orbit” which means that satellites will be going over the same spot always at the same local time. This is important for earth observation missions since existence of daylight is necessary for imaging purposes. Orbital mechanics of observation satellites is well known in practice and literature. A good source for this type of information is a book titled “Satellite Orbits: Models, Methods and Applications” by Montenbruck and Gill published in year 2000.
Due to low orbit altitude, observation satellites revolve around earth at a fast rate of 90 minute orbit time to complete a whole rotation around earth. This corresponds to enormous ground of speed of 7 km/sec or 25200 km/hr. Earth-observation satellites revolve around earth in a peculiar spiraling orbit flying over different path which is displaced few thousand kilometers from the previous path. This way, satellite revisits to the exact same spot on earth more or less in every four days.
Earth observation satellites are designed to image target areas selected by clients as they fly over the target. An image of target area with excessive cloud cover is undesirable since it obscures the details of the image. If a client requires image of a certain target area, and if that area happens to be under cloud cover at the time satellite passes over the target area, another imaging attempt has to be done at another scheduled visit. Observation satellites cannot hover over the target area waiting for cloud condition to clear, therefore, another attempt has to be done when satellite is scheduled to fly over the target area again which means few days of delay.
Observation satellites fly with high ground speed, which is about 7 km/sec. Therefore, earth observation satellite has only few second time windows over the target area, which needs to be used very efficiently. What complicates the issue further, during these few second duration window of observation, there may multiple target areas that need to be imaged. Imaging these targets require aiming main imaging instrument of the satellite toward the target area. This process is not instantaneous and requires time. Current state of the art in observation satellites is a category of satellites called “agile” satellites, which can control satellite attitude along the three axes (roll, pitch and yaw) via gyroscopic actuators. This capability allows the satellite to perform observations and transitions between observations during its flight. In other words, unlike non-agile satellites, which stand still and wait for the target area to appear under the satellite, the agile satellite can orient itself toward a target image and change its orientation toward another target area by changing its pitch, yaw and roll angles. This means non-agile satellites can image not only the areas directly underneath but also neighboring areas located on the proximity of the flight path. The difference between agile and non-agile satellite behavior is explained in FIGS. 8A and 8B. FIG. 8A shows the typical behavior of non-agile observation satellite 82. As the satellite moves along its path, it needs to select either target area 84 or target area 85 since it can only roll along its axis to image one of the target areas. In this specific case, satellite 82 images area 84 and forfeits area 85 due to lack of pitch capability during the imaging window 89. FIG. 8B shows the behavior of agile satellite 92 for the same case. Due to added pitch and yaw capability, the satellite can expand its viewing window by aiming at target 84 and then reaming at target 85 as soon as its finished with target 84. This way more target areas can be acquired during its flight.
Some technical terms needs to be explained using FIGS. 1 and 2 to explain the problem fully. In technical terms, Nadir Point 15 (Np) is called the point on the earth which is directly below the satellite at any given time during its orbit. Nadir Angle 16 (Na) is called the angle between the Satellite-Nadir point line (S-Np) and the point imaging camera is pointing at (S-Vp). In agile satellites the, the main imaging instrument can be used to image target areas that lay on the sides of the flight path as well as forward and backward targets with nadir angle +/−30°. Nadir angle of 30° degree gives possibility of imaging target areas that lay 260 km away (sideways) from the flight path of the satellite. Technically, it is possible to image targets with non-zero Nadir angle, but the quality of the image degrades as the Nadir angle increases. In other words, a target area imaged at Nadir angle 0° is preferred over an image which is imaged at Nadir angle 30°.
This “agile” capability of satellites opens up new possibilities for imaging target areas. Having capability to image areas that lay within +/−30° Nadir angle, not only on left and right of the path but also on forward and backward of the current nadir point of the satellite will present the possibility of additional target areas competing for the imaging window. Since the available time window to image is limited, it may not be possible to image all target areas. The preferences of the client are also important while deciding which target areas should be imaged. A target image with high Nadir angle may acceptable by some clients but not with some others. An image with intense cloud cover is not acceptable at all by most clients. However, a target image with 10% cloud cover may be acceptable by some clients. Therefore, it is very important that observation satellite to use its imaging time wisely to acquire image which is acceptable by the clients. Since cloud cover is the most crucial factor in earth imaging, it is important to fulfill orders with acceptable level of cloud cover among the competing orders. Studies indicate that up to 80% of images acquired by commercial observation satellites are unusable due to excessive cloud cover [1]. If imaging a target area is skipped due to some reason, it is scheduled for acquisition at another round. As the deadline set up by client for image acquisition gets closer, the priority of that particular target increases. All these additional factors also need to be taken into consideration while deciding which target images should be acquired during the imaging window.
The main payload of earth observation satellite is the imaging equipment, which is an electronic imaging camera. Since ground speed of the satellite is very high, a linear array of light-sensitive sensors are typically used for scanning the ground image making use of the forward moving motion of the satellite. The width of the earth surface which can be scanned by the satellite in one scanning action is called “swath-width” which is shown by number 10 of FIG. 1. Direction of movement of satellite 11 is shown as 14. For a typical earth observation satellite, “swath-width” is about 17 kilometers. Any target area with width larger than that, has to be scanned by repetitive scanning actions. The duration of acquisition of a particular target is an important parameter to consider since it eventually affects the available time left for other neighboring target areas.
The main payload of the modern earth observation satellites is the imaging equipment, which is typically mounted longitudinally in the center of the satellite body. In contemporary designs, the camera is fixed rigidly to the satellite body. Because of this rigid connection, the satellite body needs to be pointed toward the target area in order to capture the image of a target. Reorientation of the body of the satellite is done by gyroscopic actuators or thrusters which takes certain amount of time. Imaging a particular target has consequences on imaging other targets due to reorientation time required. The amount of activity needed by the satellite to reorient itself to a new target is called “agility requirement” and it determines the reorientation time of the satellite. Reorientation time consumes the available imaging window of the target image. Therefore, reorientation time needed by the satellite is also an important parameter to take into consideration while deciding which targets should be acquire and in which sequence.
Earth observation satellites have limited connectivity with earth stations while revolving around the globe. Typically, observation satellite connects to a ground station on its way to receive list of locations to image and continued its journey until it reaches the target destination. Upon acquiring the image of the target, image data is stored in onboard memory of the satellite until satellite reaches and other ground station on its way and downloads the image file. On the average, a typical Earth Observation satellite has ground connectivity on the 10% of the time [2]. Due to limited connectivity, satellite has to make target priority decisions on its own without waiting for operator intervention.
Execution of Image Order Requests by Satellite Operators
It is essential to understand how the operators of an earth observation satellite currently handle the image ordering requests in order to appreciate the scale of the problem fully. Typical order processing starts by customer specifying coordinates of the target area using available online tools. Online software tools help to determine the coordinates, the area and the cost of acquisition. Customer also specifies acceptable nadir angle of acquisition, acceptable cloud cover level and deadline for acquisition. All this information is required by the satellite operator to determine the priorities among competing orders to determine which orders should be given priority under different circumstances. Cloud cover information over the target locations is a crucial piece of information which determines priorities. In current practice, cloud satellites provide the cloud cover information over target. Although up-to-date cloud cover information is received regularly, local cloud cover situation is known to change quickly [3]-[4]. Another problem with the cloud satellites is the low image resolution of the cloud satellites. Due to low resolution, it is difficult to predict the actual cloud cover situation over a specific target plot.
Taking into consideration the acceptable nadir angle of the client and the deadline for acquisition, the mission control prepares an image collection plan for the journey ahead and uploads information to the satellite as soon as the connectivity is established. Satellite operator uses the historical cloud cover as well as the most recent cloud images from cloud satellites to make an educated guess of which target plots are likely to present problems and makes use of this prediction in its image acquisition schedule [5]. Ultimately, it tries to avoid potentially cloud-covered target areas for the benefit of areas with clear sky overhead. As the satellite completes its journey, images of the target areas are acquired and stored internally until a ground station is reached where the image files are downloaded. Once the images are received by the mission control, they are checked for cloud cover condition to see if they are acceptable in terms of image quality. If the cloud cover level is found to be not acceptable, another image acquisition schedule for the same target area is scheduled by the mission control for the upcoming trips of the satellite. Orbital mechanics dictate the timing of the revisit of the exact same spot, which is in the order of few days.
Issues that Complicate the Image Acquisition Process
Several issues complicate the image acquisition process in the current practice. One of them is the cloud cover, which makes the acquisition process somewhat unpredictable. Even though the mission control acquires historical cloud cover information for the target area, actual cloud cover at the instant of acquisition is what really matters [6].
Another issue that complicates acquisition process has something to do with the proximity of target plots. If target plots have sufficiently separated acquisition time windows, there is not much concern for this issue. However, if the acquisition time windows for the targets are very close, there may not be enough time to reorient the satellite to the new target.
The complication arises from the fact that image acquisition instrument is installed within the body of the satellite rigidly. The whole body of the satellite has to be oriented toward the new target area. Considering the fact that satellite is moving toward the target with ground speed in excess of 27,000 km/h, and it takes time to reorient the body of the satellite for a new image acquisition, it is easy to understand that there is limited time to maneuver the satellite of each target. The problem is explained with the aid of FIG. 4. According to the figure, there are three target areas of acquisition which are marked as A, B and C. As satellite moves toward target A, satellite body is aligned with the target with the proper nadir angle. As soon as the edge of target A appears in the main imaging equipment, acquisition starts until satellite passes over the target A. Now target B is the next target in the image acquisition sequence. However, target B requires a different nadir angle and orientation in order to acquire its image properly. Yet there is another target area C, which is in close proximity to both A and B that is competing for acquisition during this window of acquisition time. Considering the enormous ground speed and close proximity of the target areas A, B and C, there is simply not enough time to reorient the satellite to all three target areas to acquire the image. As a result, mission control analyses the historical cloud cover data and makes an educated guess to skip area B in favor of area C. For this reason, mission control may instruct the satellite to acquire images for areas A and C but not B. Complication arises from the fact that, the historical cloud information is not always accurate. The clouds move in and out in a matter of minutes, which ruins the prediction. For the sake of argument, assume that target area C is covered by cloud recently and gives a poor image. In this case, we lost the opportunity to acquire both area C and area B.
Problems like these indicate that we need a better way of getting cloud cover information so that we can make a better decision about which targets should be favored over others. Relying on historical cloud cover information, results in very low yield ratio for quality images. According to [2], this approach results in loss of 80% of images taken by the satellite.